Skin is the largest organ in a body, and a forefront for defending a living organism. Epidermis consists of keratinocytes (KC), melanocytes, epidermis Langerhans cells (LC), intraepithelial T cells, and the like.
LC are immature dendritic cells (DC), whose function is to capture and transport locally exposed protein antigen to draining lymph nodes in which acquired immune responses are generally accomplished. During their migration to a lymph node, LC develop into mature DC having antigen-presenting capacity. As a result, a systemic immune response specific to the antigen carried by the LC/DC is caused. In this way, the antigen-specific immune response in the skin is closely associated with the systemic immune response to the same antigen. Therefore, it is thought that skin and immune organs are tightly connected with each other via circulatory trooping of LC/DC and via antigen specific immune cells. On the other hand, KC and melanocytes reside in the skin, and do not principally participate in an acquired immune response.
However, KC might contribute to development of local innate immune response and local inflammation. When microbes infect skin, a host develops an inflammation reaction, and then develops an acquired immune reaction in a manner localized in the skin. At that time, KC and LC constituting the skin are closely involved in each reaction. Therefore, it is thought that, based on their unique property of producing various cytokines upon stimulation from microbes or chemical reagents, KC has a large influence on LC, with a result that KC modify an acquired immune response (see Non-Patent Documents 1 and 2). In consideration of these facts, it is important to determine whether KC-induced cutaneous inflammation can also affect the systemic immune response.
The inventor of the present invention have established caspase-1 transgenic mice (KCASP1Tg mice) which KC-specifically express caspase-1, and develop atopic dermatitis (AD)-like inflammatory skin lesions (pruritic chronic inflammation) in an interleukin 18 (IL-18)- and interleukin 1β (IL-1β)-dependent manner (see Non-Patent Documents 3 and 4). Further, the inventor of the present invention has disclosed that IL-1β enhances the capacity of IL-18 to induce AD-like inflammatory skin lesions (see Non-Patent Document 4). These results suggest that KC may also contribute to the systemic immune response by producing a number of cytokines including IL-18 and IL-1β.
The IL-18 and IL-1β are produced as biologically inert precursors, and are released as an active form after being cleaved by appropriate intracellular enzymes such as caspase-1 (see Non-Patent Documents 5 through 9).
The IL-18 has diverse biological actions depending on the kinds of cytokines coexisting with the IL-18. Particularly, in the presence of interleukin 12 (IL-12), IL-18 promotes inflammatory responses via induction of IFN-γ which is a potent pro-inflammatory cytokine (see Non-Patent Document 10). On the other hand, in the absence of IL-12, IL-18 induces atopic response via induction of production of Th2 cytokine (see Non-Patent Document 11 through 14).
The AD is an inflammatory skin lesion in response to external stimulation, and is accompanied by chronic and repetitive strong itch. The onset of the AD has a genetic basis, and a patient of the AD has high serum levels of IgE. However, the pathogenic mechanism of the AD is poorly understood. Activated T cells, basophil, and mast cells are closely involved in the pathogenic mechanism of the AD.
Particularly, it is thought that as a result of activation of mast cells or basophil by an allergen, Th2 cytokines and chemical mediators are produced, and accordingly AD is developed. The activation of mast cells or basophil by allergen is caused by crosslinking of IgE molecule bound to Fc ε R (Fc receptor (FcR) to IgE antibody of basophil) on these cells. Important ones out of the Th 2 cytokines are, for example, interleukin 4 (IL-4), interleukin 5 (IL-5), interleukin 9 (IL-9), and interleukin 13 (IL-13). Important ones out of the chemical mediators are, for example, histamine, serotonin, and leukotriene.
It is known that infection of Staphylococcus aureus exacerbates inflammation of skin of an AD patient according to environments (see Non-Patent Documents 15 and 16), and increases density of serum IL-18 of some AD patients (see Non-Patent Document 17). Namely, it is known that the infection of Staphylococcus aureus is an inducing factor or exacerbation factor of AD. However, it is poorly known how Staphylococcus aureus is involved in the onset of AD.
[Non-Patent Document 1]
Jamora, C. and Fuchs, E. 2002. Intercellular adhesion, signaling and the cytoskeleton. Nat. Cell Biol. 4:E101
[Non-Patent Document 2]
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[Non-Patent Document 3]
Yamanaka, K., Tanaka, M., Tsutsui, H., Kupper, T. S., Asahi, K., Okamura, H., Nakanishi, K., Suzuki, M., Kayagaki, N., Black, R. A., et al. 2000. Skin-specific caspase-1 transgenic mice show cutaneous apoptosis and pre-endotoxin shock condition with a high serum level of IL-18. J. Immunol. 165:997.
[Non-Patent Document 4]
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[Non-Patent Document 17]
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At present, the pathogenic mechanism of AD is poorly understood, and therefore it is very difficult to develop an effective therapeutic drug for AD.
As described above, it is known that cells constituting epidermis are involved in a variety of immune responses. For example, it is recently reported that dendritic cells induce an acquired immune response as antigen presenting cells. However, as for KC which are the most main cells out of cells constituting epidermis, it is not clear how KC are involved in immune response of a host.
In developing an effective therapeutic drug for a given disease, it is one of important methods to understand the pathogenic mechanism of the disease, and to perform, by use of the mechanism, screening of a substance having a pharmacological effect. However, as for the onset of AD, there are a lot of unsolved points including involvement of KC and infection of Staphylococcus aureus. Therefore, a technique (including screening) for applying them to the development of a therapeutic drug for AD is rarely known.
The present invention is made in view of the foregoing problems, and its object is to provide, by means of an induction phenomenon of generation of IL-18 from KC, a variety of methods preferably applicable to solution of the pathogenic mechanisms of atopic dermatitis and atopic dermatitis-like symptoms, and for development of a therapeutic drug for atopic dermatitis and atopic dermatitis-like symptoms, and usages of the methods.